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<H1>head_queue(Queue, Head)</H1>
unifies Head with the first element of the queue
<H2>Description</H2>

    unifies Head with the first element of the queue.  The tricky part
    is that we might be at the end of a queue: Back-Back, with Back a
    variable, and in that case this predicate should not succeed, as we
    don't know what that element is or whether it exists yet.
    

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